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<jats:title>ABSTRACT</jats:title><jats:p>Seminal fluid proteins (SFPs) exert potent effects on male and female fitness. Rapidly evolving and molecularly diverse, they derive from multiple male secretory cells and tissues. In <jats:italic>Drosophila melanogaster</jats:italic>, most SFPs are produced in the accessory glands, which are composed of ∼1000 fertility-enhancing ‘main cells’ and ∼40, more functionally cryptic, ‘secondary cells’. Inhibition of BMP-signalling in secondary cells suppresses secretion, leading to a unique uncoupling of normal female post-mating responses to the ejaculate: refractoriness stimulation is impaired, but offspring production is not. Secondary cell secretions might therefore make a highly specific contribution to the seminal proteome and ejaculate function; alternatively, they might regulate more global – but hitherto-undiscovered – SFP functions and proteome composition. Here, we present data that supports the latter model. We show that in addition to previously reported phenotypes, secondary cell-specific BMP-signalling inhibition compromises sperm storage and increases female sperm use efficiency. It also impacts second male sperm, tending to slow entry into storage and delay ejection. First male paternity is enhanced, which suggests a novel constraint on ejaculate evolution whereby high female refractoriness and sperm competitiveness are mutually exclusive. Using quantitative proteomics, we reveal a mix of specific and widespread changes to the seminal proteome that surprisingly encompass alterations to main cell-derived proteins, indicating important cross-talk between classes of SFP-secreting cells. Our results demonstrate that ejaculate composition and function emerge from the integrated action of multiple secretory cell-types suggesting that modification to the cellular make-up of seminal fluid-producing tissues is an important factor in ejaculate evolution.</jats:p>